Control of an internal combustion engine is achieved by manipulation of one or a plurality of actuators. For example, in the case of control of a spark ignition type internal combustion engine, actuators such as a throttle, an ignition device and a fuel supply device are manipulated. The manipulation variables of the plurality of actuators may be individually determined for each of the actuators. However, with use of the torque demand control as disclosed in Japanese Patent Laid-Open No. 10-325348, control precision of torque can be enhanced by cooperative control of a plurality of actuators.
Torque demand control is a kind of feed forward control which uses torque as the control variable of an internal combustion engine, and determines the manipulation variable of each of the actuators so as to realize a required value thereof. In order to execute torque demand control, a model for deriving the manipulation variable of each of the actuators from a torque required value, in more detail, an inverse model of the internal combustion engine is needed. An engine inverse model can be configured by a map, a function or the combination of them. Japanese Patent Laid-Open No. 10-325348 discloses the art of enabling torque demand control by using a common model (expressed as control target amount calculation means in the above described publication) at an idle time and a non-idle time of an internal combustion engine.
Incidentally, the relationship between the manipulation variable of each of the actuators in an internal combustion engine and torque which is a control variable changes in accordance with the operating state and the operation conditions of the internal combustion engine. Accordingly, in order to calculate the manipulation variable of each of the actuators for realizing a torque required value accurately, an operating state and operation conditions are required as information. However, depending on the situation in which an internal combustion engine is placed, the necessary information cannot be sometimes obtained. For example, the air quantity which is taken into a cylinder can be calculated by using a throttle opening and the output value of an air flow sensor, but at the time of start, air already exists in an intake pipe, and therefore, calculation of an accurate intake air quantity is difficult. When the reliability of the engine information for use in torque demand control is low, each of the actuators cannot be properly manipulated, and control precision of torque cannot be ensured.
As one idea for coping with such a situation, directly instructing individual actuators about the manipulation variables is conceivable in place of determining the manipulation valuable of each of the actuators from a torque required value. If instruction of the manipulated variables of the actuators can be directly given, even if the reliability of the engine information is low, at least unintended manipulation of the actuators is prevented from being performed.
Further, it is also effective to enable direct instruction of the manipulation variables of the actuators in the case of performing special control which is not assumed in the engine inverse model. For example, an internal combustion engine exists, which enables operation by homogeneous combustion at a time of a middle and high load and operation by stratified combustion at a time of a low load. However, the relationship of the manipulated variable of each of the actuators and torque which is a control variable totally differs between homogenous combustion and stratified combustion. Therefore, when the aforementioned engine inverse model is designed with homogenous combustion as a precondition, the manipulation variables of the actuators cannot be calculated by using the engine inverse model at the time of stratified combustion. In such a case, if direct instruction of the manipulation variables of the actuators is possible, each of the actuators can be operated with the manipulation variable corresponding to the stratified combustion.
As described above, as the setting method of the manipulation variables of the actuators, there are the method which sets the manipulation variable with the required value of the physical quantity such as torque used as information, as the conventional torque demand control, and the method which sets the manipulation variables by direct instruction to the individual actuators. The former method has the advantage of being capable of operating respective actuators while allowing them to corporate with each other for realization of the requirement concerning the physical quantity. The latter method has the advantage of being capable of causing each actuator to execute the necessary operation in control of an internal combustion engine properly without receiving the influence of the operating state and the operation conditions of the internal combustion engine. Like this, both the methods have their own advantages, but have disadvantages. However, the advantage of one is in the complementary relationship with the disadvantage of the other, and therefore, in making both of them properly switchable, a large merit can be expected in control of an internal combustion engine.
However, one problem exists here. This is, in what timing switching is performed. Since the physical quantity such as torque depends on the manipulation variables of actuators, if switch timing is not proper, discontinuity is likely to occur to any of the physical quantities. For example, in the case of occurrence of discontinuity in torque, reduction in drivability due to torque shock is brought about.